1. Field of the Invention
This invention relates to an apparatus for taking up an object space using an opto-electronic range finder, which measures individual distances according to a time-of-flight of signal method. More particularly the invention relates to such an apparatus which comprises a transmitter for transmitting optical transmitter signals in the form of a beam of a predetermined angle of beam along an optical axis, and a first receiver having an optical axis for receiving optical signals reflected from an object situated in the object space after a time-of flight of the transmitter signals and the reflected signals which, thus, have a certain time relationship. The optical signals transmitted and received after reflection will preferably be laser signals.
For determining range values from either the time-of-flight or time relationship (phase relationship), there is an evaluating stage which compares the transmitter signals and the reflected signals.
In addition to the above characteristics, the apparatus comprises a second receiver for receiving electromagnetic radiation to form an image from individual image elements, and a scanning device for deviating the direction of said optical axes of said transmitter means as well as of said first and second receiver means in two directions and by a certain deviation angle per said individual distance measurements so as to ensure that the directions of the optical axes of said first and second receiver means coincide and scan said object space synchronously and in phase. Scanning is preferably done in two directions orthogonal to each other, although the invention is not restricted to that.
When, in this context, a “scanning device” is mentioned, it is possible within the frame of the present invention that this scanning device comprises separate scanners for each of the above mentioned axes or at least two.
2. Background Information
Known devices of this kind produce so-called “distance images” or point clouds of the object space.
A possible visualization of the data, thus obtained, is given by using so-called mock colors wherein defined colors are assigned to certain distance ranges. In spite of such measures, interpreting such images is often quite difficult. This applies, in particular, to taking buildings, because their many surfaces can be intensively structured; since, however, these structures will either not protrude from the surface or protrude only by a small amount, they can not, or can hardly, be recognized in the “distance image”. This is particularly disturbing, if one wants to generate a 3-D computer model of the object from a plurality of individual images, because the data will not contain sufficient information about the structure of the surfaces.
Similar problems will arise, when such an apparatus provided with a scanner should be used for surveying a place after a traffic accident: while, in general, the vehicles involved, the roadside borders as well as the position of traffic signals can be reproduced very well, it is not possible to show wheel traces or marks on the road used to determine the position at crash of the vehicles involved or of victims of the accident. However, such traces and marks are of essential importance for documenting the way the accident happened.
In order to solve these problems, it has already been suggested, when scanning a scene with the range finder, to scan it synchronously and in phase with an opto-electronic receiver for electromagnetic radiation. In the U.S. Pat. No. 5,528,354, a laser beam was directed to the object and the reflected light was directed both to a range finder portion and an image forming portion. However, just in the narrow wave range of laser light, various items and characteristics remain invisible so that the center of the problem has only been shifted.
It has also been suggested to use, apart from the actively illuminated range finder channel, a purely passive channel, particularly using visible light. In this case, an optical system has been pre-posed to the receiver and adapted the image angle of the receiver approximately to the angle of beam of the transmitter's beam of the range finder. From the corresponding signals, thus obtained, image elements are determined, each image element having a spatial angle assigned (see WO 99/46 614). The above-mentioned receiver may be a color receiver, a monochromatic receiver (black and white) or an infrared receiver. The result when taking up a scene is the above-mentioned “distance images” or very realistic 3-D computer models which show all details and designs on the various surfaces so that an interpretation of these images is relatively simple and additional surveying work, e.g. when documenting a traffic accident, can be avoided.
When using such systems, difficulties can arise when the object is only insufficiently illuminated, as is normally the case when taking inner rooms of large buildings, e.g. churches, halls or when taking caverns, tunnels or caves. Since the aperture of the optical system is limited for constructive reasons and the time of a complete scanning cycle cannot be prolonged at will, the object space, in critical cases, had to be illuminated with searchlights which, of course, is rather troublesome.
In order to mitigate these difficulties, it has been suggested to scan the object space with a sharply directional light beam or light fan in synchronization with the scanning device (see AT-Patent No. 409 195).
The problem described above is still aggravated as the users of such scanning systems are interested in increasing the resolution in order to be able to display more and more details of a combination of a “distance image” (cloud of measuring points) and a visible image or of a 3-D model. However, the resolution of such systems is limited by the divergence of the transmitted laser beam. The divergence of the transmitted beam of appliances according to the prior art amounts about 3 mrad. Modern lasers, in contrast, enable realizing an angle of beam of about 0.3 mrad, thus providing the basis for high-resolution scanning systems. In systems which, apart from a “distance image” supply also a parallel color image or a black-and-white image by a passive sensor (without illumination by the apparatus), the requirement will result to generate also the latter one with the same resolution. However, a reduction of the image angle of a passive sensor in the known designs is only possible by reduction of the light sensitive area of the sensor which leads to a clear reduction of sensitivity.